The IEEE 802.16 communication standard, or WiMAX, uses an Orthogonal Frequency Division Multiple Access (OFDMA) protocol. In the OFDMA system, a mobile station (MS) is assigned a frequency sub-channel and a time slot in a physical layer for its communications with a base station, node B, or access point (AP). It is important in an OFDMA system to maintain both time and frequency synchronization. If frequency synchronization is lost then orthogonality between the various sub-carriers assigned to other MSs is also lost, which results in interference between MSs. If time error is present, system performance will be degraded due to received signal constellation rotation. Therefore, it is required in WiMAX that individual MSs maintain time and frequency synchronization with an AP to which the MSs are connected. However, synchronization becomes problematic in some WiMAX operational modes, such as uplink spatial division multiple access (UL SDMA) mode for example.
UL SDMA is a mandatory feature of WiMAX that can theoretically double system capacity by having two different subscribers (with single antenna and transmitting independent data streams) share the same time/frequency resource. To facilitate channel estimation at the receiver side, the WiMAX standard has specified pilot structures in a Partial Usage of Subchannels (PUSC) tile, as shown in FIG. 2. This single tile has four pilot structures in each corner. The pilots are well designed such that they are orthogonal from the base station perspective, where a channel estimate for one subscriber can be calculated without interference from the other sharing subscriber. Ordinarily, for this PUSC signal structure, pilot-based timing and frequency errors are estimated by calculation of pilot signal phase ramp across a time dimension (e.g. OFMD symbol index) and a frequency dimension (e.g. tone index) respectively based on embedded pilot signals.
However, in a UL SDMA implementation, two subscribers share the same tile such that each subscriber uses only two pilots at diagonally opposite corners of the tile, while the remaining (null) corners are not used by that subscriber. The data subcarriers are then shared between the subscribers. For example, referring to FIG. 3, one shared subscriber is assigned to use the corner pilots in pattern A, and the other shared subscriber is assigned to use the other corner pilots in pattern B. However, a problem arises in that this pilot structure together with tile hopping in the uplink makes receiver synchronization very difficult because there are only two pilot signals used for frequency and time error estimation, and these pilot signals are not on the same tone or the frequency separation is too large on the same OFDM symbol index. In other words, there are no two pilots on the same tone for frequency error estimate or the frequency separation within one OFDM symbol is too large for a timing error estimation. This inherent limitation of the pilot structure of a PUSC tile limits UL SDMA application, particularly in the case of high velocity of a subscriber, which introduces degradation problems associated with the Doppler shift (frequency error) and timing errors caused by fast moving.
Accordingly, what is needed is a technique to alleviate the degradation that occurs when using the same WiMAX tile pilot structure for all transmitted UL tiles for a subscriber, such as in UL SDMA mode. It would also be of benefit to minimize the effects of Doppler (frequency) and timing errors.
Skilled artisans will appreciate that common but well-understood elements that are useful or necessary in a commercially feasible embodiment are typically not depicted or described in order to facilitate a less obstructed view of these various embodiments of the present invention.